The present invention relates to a method for increasing the firing range of air-defence and artillery shells of the type which for reasons concerning firing technology have a rear surface which is at right angles to the shell axis, i.e. in principle all conventional air-defence and artillery shells. The invention also includes a design of air-defence and artillery shells that corresponds with the said method.
It is well known that the rear surface that is at right angles to the axis of the shell while being desirable at firing, has an adverse affect on the shell in the case of long-range firing due to the airflow along the rear edge of the shell causing turbulence which in its turn results in low pressure and drag to the rear of the shell thus limiting the maximum range of the shell.
The base-bleed technique has been much used in recent years to increase the range of air-defence and artillery shells without having to increase muzzle velocity and thereby the size of the propellant charge to a level which the gun in question would not withstand.
The base-bleed technique is based on allowing gas to flow out from the rear surface of the shell preferably during the release of heat, and the flow rate of the gas in such a case shall be at a level that enables the gas flow to eliminate most of the low pressure and the drag caused. Although the base-bleed device is similar to a supplementary rocket motor with its propellant loaded interior chamber and its central flow outlet, its function is totally different from that used in shells which are fitted with supplementary rocket motors known as sustainers to increase firing range. Such rocket motors are loaded with pure rocket propellant and they provide the shell with a velocity increment, while the base-bleed device is loaded with a slow burning propellant which is intended only to eliminate drag during the portion of the shell trajectory the propellant is burning. One of the advantages of the base-bleed technique is that the base-bleed device requires considerably less space in the shell than that required by a supplementary rocket motor, but the base-bleed device does take up space in the shell which could be used for an active load. The base-bleed device futhermore represents a not inconsiderable increase in cost.
The present invention now offers a simpler, cheaper and less space-consuming solution to the problem of eliminating or at least minimising the disturbing and range-reducing low pressure at the rear of air-defence and artillery shells which at the time of firing have the rear surface at right angles to the axis of the shell.
In accordance with the principle of the invention, the rear surface of the shell which is at right angles to the shell axis is given a more aerodynamically advantageous form as soon as the shell leaves the barrel of the artillery gun from which it is fired. This can be achieved for example by extending the shell at the rear by a protruding conical tail section. The said tail section can thereby consist of an inflatable part initially fitted in the rear section of the shell in compressed form and secured to the shell body, and can be folded out and inflated to the desired form and hardness by the propellant gases from a small propellant charge which is ignited at the required time. Such an inflatable section part can for example be made of Kevlar and remain in a removable cover connected to the shell up to the time it is deployed.
Another conceivable version of the invention is based on the rear surface of the shell itself consisting of a disposable cover which is removed from the shell as soon as the shell has left the barrel, upon which the actual shell rear section is revealed and this has an aerodynamically favourable conical form narrowing progressively to the rear. In this case also, a smaller propellant charge can be used for separating the cover in question.
Both of the basic versions of the invention are now described in more detail in combination with the enclosed figures while the invention in its entirety is defined in the subsequent patent claims.